Thanks guys.
LM, look in your inbox.
Rob, I sat there for a while. They walk to the end of the walk, turn, stand there for a minute or two, move the rifle from one shoulder to the next, then march back the other way and do it again. So, I waited several times, and used burst mode to get the money shot. FYI, the guard for the Unknown Soldier is one of the only ones in the US that are like the guards at Buck palace, ignoring the people around them.